1. Field of the Invention
The present invention relates to a peeling method of a peeled off layer, especially, a peeling method of a peeled off layer containing various elements. In addition to that, the present invention relates to a semiconductor device comprising a circuit composed of a thin film transistor (hereinafter referred to as TFT) transferred by sticking a separated peeled off layer to a substrate and its manufacturing method. The invention relates, for example, an electrooptical device such as a liquid crystal module, a light emitting device such as an EL module, and an electronic appliance containing such devices as components therein.
The term semiconductor device, in this specification, covers any device as a whole capable of functioning by utilizing semiconductor characteristics and an electrooptical device, a light emitting device, a semiconductor circuit, and electronic apparatus are all included in the semiconductor device.
2. Related Art
Recently, attention has been paid to a technology of composing a thin film transistor (TFT) using a semiconductor thin film (the thickness of about several to several hundred nm) formed on a substrate having an insulating surface. The thin film transistor is widely applied to electronic devices such as IC and an electrooptical device. Especially, its development as a switching element for an image display apparatus has been urged.
Among a variety of expected applications of such an image display apparatus, above all, utilization for mobile appliances draws attention. Today, although a glass substrate, a quartz substrate and the like are used in many cases, they have disadvantages that they are easy to crack and heavy. Further, in terms of mass production, the glass substrate, the quartz substrate and the like are difficult and unsuitable for enlargement. Therefore, it is tried to use a substrate having flexibility, such as a flexible plastic film to form a TFT element thereon.
However, since the heat resistance of the plastic film is low, the highest temperature for process is required to be low and consequently, it is presently impossible to form any TFT with electric characteristics as excellent as those formed on a glass substrate. Hence, no liquid crystal display apparatus and light emitting element using a plastic film has been actualized.
Further, a peeling method for peeling from the said substrate a peeled off layer existing on a substrate through an isolation layer, has been already proposed. For example, a technique described in Japanese Unexamined Patent Publication No. 10-125929 and Japanese Unexamined Patent Publication No. 10-125931 is a method for separating a substrate by forming an isolation layer of amorphous silicon (or polycrystalline silicon) and releasing hydrogen contained in the amorphous silicon by radiating laser beam through a substrate to form voids for substrate separation. In addition to that, in Japanese Unexamined Patent Publication No. 10-125930, there is a description that using this technique, a liquid crystal display apparatus is completed by sticking a peeled off layer (in the specification, called as a transferred layer) to a plastic layer.
However, by the above-described method, it is essential to use a substrate with high translucency. In order to transmit energy through the substrate and supply energy sufficient to release hydrogen contained in the amorphous silicon, relatively intense laser beam has to be radiated, resulting in a problem that the peeled off layer is damaged. Further, by the above-described method, in the case an element is formed on an isolation layer, if heating treatment at a high temperature is carried out in the element fabrication process, hydrogen contained in the isolation layer is diffused and decreased. As a result, there is a possibility of insufficient peeling-off, even if laser beam is radiated to the isolation layer. Subsequently, attributed to retention of the quantity of hydrogen contained in the isolation layer, there occurs a problem that the process after isolation layer formation may be limited. Also, in the above-described specification, there is a description that in order to prevent damages in the peeled off layer, a light shielding layer or reflecting layer is formed. In such a case, it becomes difficult to fabricate a transmission type liquid crystal display apparatus. Additionally, by the above-described method, peeling of the peeled off layer having a large surface area becomes difficult.